I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for performing handoff synchronization between synchronous and asynchronous base stations in a CDMA wireless communication system.
II. Description of the Related Art
FIG. 1a is an exemplifying embodiment of a wireless communication system having a terrestrial wireless network 140 in communication with a wireless mobile station 110. Wireless network 140 is shown with two wireless base stations 120 and 130 such that mobile station 110 can communicate with either and can handoff between the two. The wireless communication channel through which information signals, travel from mobile station 110 to a base station 120 or 130 is known as a reverse link. The wireless communication channel through which information signals travel from a base station 120 or 130 to mobile station 110 is known as a forward link.
Though only one serving base station is shown, an MS may communicate with multiple serving base stations in soft handoff and may establish a handoff to multiple target base stations. Mobile station 110 communicates exclusively with one or more serving base stations 120 before handoff is established. After handoff is established, mobile station 110 may communicate in "soft handoff" with both serving base stations 120 and target base stations 130. Alternatively, following a "hard handoff," mobile station 110 communicates exclusively with target base stations 130. In reality, typical wireless communication systems may have many more mobile stations and base stations than shown. Wireless network 140 includes one or more base station controllers or BSC's (not shown) and one or more mobile switching centers or MSC's (not shown). Serving base station 120 may be connected to a different BSC and MSC than target base station 130, or the two base stations may share the same BSC and MSC.
Mobile station 110 may be any of a number of different types of wireless communication devices such as a portable phone, a wireless communication module incorporated into a portable computer, or a fixed location communication module such as might be found in a wireless local loop or meter reading system. In the most general embodiment, mobile stations may be any type of communication unit. For example, the mobile stations can be hand-held personal communication system units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment.
In an exemplary embodiment, mobile station 110 communicates with serving base station 120 and target base station 130 using code division multiple access (CDMA) techniques. An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TIA/EIA Interim Standard entitled "Mobile station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA/IS-95, and its progeny (collectively referred to herein as IS-95), the contents of which are also incorporated herein by reference. More information concerning a code division multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated in its entirety herein by this reference.
Third-generation CDMA wireless communications systems have also been proposed. The cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission proposal forwarded by the Telecommunications Industry Association (TIA) to the International Telecommunication Union (ITU) for consideration for the IMT-2000 CDMA standard is an example of such a third-generation wireless communication system. The standard for cdma2000 is given in draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. For example, in both the cdma2000 and IS-95 systems, each base station time-synchronizes its operation with other base stations in the system. Typically, the base stations synchronize operation to a universal time reference such as Global Positioning Satellites (GPS) signaling; however, other mechanisms can be used. Based upon the synchronizing time reference, each base station in a given geographical area is assigned a sequence offset of a common pseudo noise (PN) pilot sequence. For example, according to IS-95, a PN sequence having 2.sup.15 chips and repeating every 26.67 milliseconds (ms) is transmitted as a pilot signal by each base station. The pilot PN sequence is transmitted by each base station at one of 512 possible PN sequence offsets. Each base station transmits the pilot signal continually, which enables mobile stations to identify the base station's transmissions as well as for other functions.
In addition to the pilot PN sequence, each base station in an IS-95 system transmits a "sync channel" signal that is synchronized to the pilot PN signal. The sync channel contains information such as the PN offset of the base station and the CDMA System Time used by the base station. When an IS-95 mobile station first powers on, it must obtain the CDMA System Time from an IS-95 base station before it can communicate with any synchronous base station. To obtain CDMA System Time, an IS-95 mobile station performs a two-step acquisition procedure. First, the mobile station performs a full search for the pilot PN sequence transmitted by any synchronous base station. This search may be performed over the entire 26.67 millisecond pilot PN code space. Upon locating a pilot PN signal, the mobile station then decodes a sync channel message from that base station's sync channel. Once a mobile station successfully reads the sync channel, the mobile station has a reference CDMA System Time that is accurate to within the time it takes for the base station's signal to reach the mobile station. Because this time is dependent on the unknown length of the signal propagation path between the mobile station and the base station, it is called the path delay uncertainty. Because the path delay uncertainty is very small compared to the 512 possible pilot PN offsets used in IS-95, the mobile station can unambiguously distinguish signals transmitted by different base stations by their PN offsets.
Base station time-synchronization as provided in the cdma2000 and IS-95 systems has many advantages with respect to system acquisition and handoff completion time. Synchronized base stations and time-shifted common pilot signals as discussed above permit a fast one-step correlation for system acquisition and detection of neighboring base stations. Once the mobile station has acquired one base station, it can determine system time that is the same for all neighboring synchronous base stations. In this case, there is no need to adjust the timing of each individual mobile station during a handoff between synchronous base stations. Additionally, the mobile station does not need to decode any signal from the new base station in order to obtain rough timing information prior to handing off.
Another recently-proposed 3G communication system is referred to as W-CDMA. One example of a W-CDMA system is described in the ETSI Terrestrial Radio Access (UTRA) International Telecommunications Union (ITU) Radio Transmission Technology (RTT) Candidate Submission forwarded by ETSI to the ITU for consideration for the IMT-2000 CDMA standard. The base stations in a W-CDMA system operate asynchronously. That is, the W-CDMA base stations do not all share a common universal time reference. Different base stations are not time-aligned. Consequently, a W-CDMA base station may not be identified by its pilot signal offset alone. Also, once the system time of one base station is determined, this cannot be used to estimate the system time of a neighboring base station. For this reason, mobiles in a W-CDMA system use a three-step PERCH acquisition procedure to synchronize with each base station in the system. Each step in the acquisition procedure identifies a different code within a frame structure called a PERCH channel.
FIG. 1b illustrates the parts of a frame transmitted on the W-CDMA PERCH channel. The PERCH channel is transmitted by each base station in a W-CDMA communication system and permits mobile stations to acquire synchronization with each base station. A frame is 10 milliseconds in duration and consists of 40,960 chips. A frame is divided into 16 slots, each slot having 2560 chips. Each slot can then be thought of as being divided into 10 consecutive parts, each part consisting of 256 chips. For the purposes of this disclosure, the 10 parts of each slot are numbered from 1 to 10, with 1 being the earliest transmitted 256 chips of each slot.
The first 256 chips (part 1) of each slot in the frame consist of two synchronization codes transmitted simultaneously. The first of the two codes is the primary synchronization code (PSC) sequence. The PSC sequence is the same sequence of 256 chips for every slot and for every base station in a W-CDMA system. The second of the codes transmitted in part 1 is the secondary synchronization code (SSC). The SSC sequences identify the timing of the parts in each slot, as well as the code group to which the transmitting base station belongs.
Parts 2 through 6 of each slot include broadcast data such as the system identity of the transmitting base station and other information that is of general use to all mobile stations in communication with that base station. Parts 7 through 10 of each slot are used to carry a pilot code signal that is generated in accordance with an Orthogonal Gold code as defined by the aforementioned UTRA specification.
Mobile stations in a W-CDMA system employ a 3-step acquisition procedure, in which each step requires multiple parallel correlations to identify the timing of different synchronization codes. The mobile station first synchronizes to the PSC to identify the boundaries between the different parts of each slot. Second, once the boundaries between parts have been identified, the mobile station uses the timing information derived from the primary synchronization channel to derive slot timing information and a Group ID from the secondary synchronization channel. Third, the mobile station decodes the pilot code signal in order to determine the identity of the transmitting base station and to identify the boundaries between frames. Once this process is complete, the mobile station may also be required to decode information from the broadcast channel (BCCH) to unambiguously determine the system time of the transmitting base station. The BCCH transmits timing information that can be used to identify the W-CDMA frame number. Because the frame boundaries are aligned with W-CDMA System Time, the W-CDMA frame number combined with the frame timing information can be used to determine the W-CDMA System Time of a W-CDMA base station.
Recently, a combined CDMA IMT-2000 standard has been proposed in which cdma2000-compliant equipment and W-CDMA-compliant equipment may be optionally supported by any manufacturer. Thus, it is expected that synchronous base stations of a cdma2000-compliant system will be geographically located near asynchronous base stations of a W-CDMA-compliant system. This creates a need to be able to handoff a mobile station that supports both cdma2000 and W-CDMA operation between the asynchronous base stations of a W-CDMA system and the synchronous base stations of a cdma2000 system, and vice versa.
U.S. Pat. No. 5,267,261 entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM," which is assigned to the assignee of the present invention and which is incorporated herein, discloses a method and system for providing communication with the mobile station through more than one base station during the handoff process. Further information concerning handoff is disclosed in U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", U.S. Pat. No. 5,640,414, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", and U.S. Pat. No. 5,625,876 entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION," each of which is assigned to the assignee of the present invention and incorporated in its entirety herein by this reference. The subject matter of U.S. Pat. No. 5,625,876 concerns so-called "softer handoff." For the purposes of this document, the term "soft handoff" is intended to include both "soft handoff" and "softer handoff."
Each base station is associated with a set of neighboring base stations surrounding the base station. Due to the physical proximity of the coverage areas of the neighboring base stations to the coverage area of the active base station, the mobile stations which are communicating with the active base station are more likely to handoff to one of the neighboring base stations than to other base stations in the system. In the IS-95 and cdma2000 systems, the base station identifies the neighboring base stations to the mobile stations with which it has established communication using a neighbor list message. The neighbor list message identifies a neighboring base station according to the PN sequence offset at which it transmits the pilot signal. In the IS-95 and cdma2000 systems, there is a one-to-one correspondence in a given geographical area between a base station and a PN sequence offset. In other words, two base stations operating in the same geographical area do not both use the same PN sequence offset. Thus, a base station in the IS-95 or cdma2000 system can be uniquely identified in a geographical region by its PN sequence offset.
The mobile station uses the neighbor list to limit the space over which it searches for handoff candidates. Because the searching process is so resource intensive, it is advantageous to avoid performing a search over the entire set of possible PN sequence offsets. By using the neighbor list, the mobile station can concentrate its resources on those PN sequence offsets which are most likely to correspond to useful signal paths.
A typical IS-95 or cdma2000 neighbor acquisition operation is practical so long as each base station's timing remains synchronous with respect to the others. However, in some systems such as W-CDMA, advantages are achieved by decoupling operation of the system from a synchronizing reference. For example, in a system which is deployed underground, such as in a subway system, it can be difficult to receive a universal time synchronization signal using GPS. Even where strong GPS signals are available, it is perceived as desirable in some political climates to decouple system operation from the U.S. Government GPS system. There may be other reasons for decoupling operation of the system from a synchronizing reference.
In a system where one or more of the base stations operate asynchronously with respect to other base stations in the system, the base stations cannot be readily distinguished from one another based merely upon a relative time offset (typically measured as a relative PN sequence offset) because a relative time offset between the base stations cannot be established without the use of a universal time reference. Thus, when a mobile station is in communication with an asynchronous base station, and has not been recently in communication with a synchronous base station, the mobile station is unlikely to have system time information of the synchronous base stations to a sufficient accuracy.
For example, suppose a mobile station has been in the coverage area of an asynchronous base station and is moving into the coverage area of a synchronous base station. Further suppose that the mobile station is able to detect the pilot signals of two different synchronous base stations by determining their relative PN sequence offsets. Unless the mobile station already knows system time of the synchronous base stations to a sufficient accuracy, the mobile station may not be able to distinguish the pilot signal of one base station from the pilot signal of another. For example, even if the mobile station can determine the received pilot signals belong to two synchronous base stations, the mobile station may be unable to identify either based on their pilot signals alone.
In a conventional IS-95 or cdma2000 system, once the forward pilot channel is acquired, the mobile station can then demodulate the forward synchronization channel. This is possible because the forward sync channel timing is such that its frame boundary is always aligned to the beginning of the PN sequence of the forward pilot channel. In other words, the forward sync channel frame boundary is always offset from system time by the same number of PN chips as the PN sequence offset of the corresponding forward pilot channel. The forward sync channel carries a sync channel message which includes overhead information such as system identification, system time, the base station's PN sequence offset, and several other items of useful information. After demodulating the sync channel message, the mobile station adjusts its internal timing according to the PN offset and system time sent in the sync channel message as described in IS-95.
The conventional sync channel is transmitted at a low data rate (for example, 1200 bps in IS-95), and the sync channel message contains a large amount of overhead information that must be demodulated on a frame-by-frame basis. Consequently, determining the system identity of the transmitting base station via the sync channel message may take as long as 800 milliseconds. This delay can undesirably affect the timing of a handoff from the asynchronous base station to the synchronous base station, particularly in a fading environment. In some instances, the delay associated with the mobile station having to determine the system identification of the synchronous target base station(s) by demodulating a conventional sync channel message would be unacceptably long, causing degradation or even dropping of a call in progress.
In a cdma2000 system, a mobile station communicating with a serving base station can quickly and efficiently search for signals of neighboring base stations. Once a mobile station receives a neighbor list message from the serving base station, the mobile station can determine a time offset search window within which the signal transmitted from a target base station will arrive at the mobile station. Because the width of the search window is roughly proportional to the average time needed to locate a signal within the window, a narrow search window is preferable to a wide search window. In a cdma2000 system, the narrowness of the search window is limited only by the relatively small path delay uncertainty between the mobile station and the two different base stations. Additionally, once the mobile station in a cdma2000 system finds the signal of the target base station within the search range, the mobile station can uniquely identify the target base station by its PN offset.
In contrast, in a W-CDMA system in which the base stations may not be synchronized with each other, a mobile station cannot use timing information from a serving base station to accurately predict the arrival time of a signal from a target base station. In order to locate the signal of an unsynchronized target base station, a W-CDMA mobile station must perform the full three-step PERCH acquisition procedure described above. This acquisition procedure takes substantially longer than searching within a narrow window for a cdma2000 signal, and therefore increases the time required to establish a handoff in a W-CDMA system.
In the worst case, the timing of different W-CDMA base stations may vary so widely that the mobile station will not know the transmit frame number of the target base station even after performing the three-step PERCH acquisition procedure. This ambiguity can occur if the base stations within the W-CDMA wireless system might be as much as a half-frame period out of synchronization. In such a system, the mobile station must monitor information transmitted on the target base station's broadcast channel to identify the frame number of the target base station. This extra decoding step further increases the time required to establish a handoff in a W-CDMA system.
The lack of synchronization in asynchronous systems such as W-CDMA systems presents difficulties in establishing handoff between synchronous and asynchronous base stations. Such problems arise when a mobile station is located in a region that lies within the coverage area of both synchronous and asynchronous wireless systems. Such a region can exist along the border between a synchronous and an asynchronous system, or may exist throughout a coverage area serviced by overlapping synchronous and asynchronous wireless systems, such as W-CDMA and cdma2000 systems.
In order to perform handoff from an asynchronous system to a synchronous system, a mobile station must hand off from an asynchronous base station to a synchronous base station. Because the asynchronous serving base station, unlike the synchronous target base station, is not synchronized to a universal time reference, the asynchronous base station cannot provide timing information for the target base station to the mobile station. In the absence of timing information for the synchronous target base station, the mobile station must perform a full pilot search and then read the sync channel in order determine the system timing of the target base station. Performing a full pilot search and reading the sync channel of the target base station is undesirable, because it increases the time required to establish a handoff.
In order to perform handoff from a synchronous system to an asynchronous system, a mobile station must hand off from a synchronous base station to an asynchronous base station. Because the asynchronous target base station, unlike the synchronous serving base station, is not synchronized to a universal time reference, the synchronous base station cannot provide timing information for the target base station to the mobile station. In the absence of timing information for the asynchronous target base station, the mobile station must perform the full three-step PERCH acquisition procedure to determine frame timing of the asynchronous target base station. In order to find the full system time of the asynchronous base station, the mobile station must then read the broadcast channel information transmitted by the asynchronous target base station. Performing the three-step PERCH acquisition procedure and reading the broadcast channel of the target base station is undesirable, because it increases the time required to establish a handoff.
Thus, there is a need for an improved method and system for facilitating handoff between asynchronous and synchronous base stations that avoids the undesirable delays associated with determining the system time of the target base station.